Chapter 24: Transition Elements

The core focus of this chemistry chapter is the comprehensive study of transition elements, defined as d-block elements that form at least one stable ion possessing an incomplete d sub-shell, thereby excluding elements like Scandium and Zinc. A fundamental characteristic of these elements is their ability to exhibit variable oxidation states due to the similar energy levels of their 3d and 4s atomic orbitals, allowing them to lose electrons from the 4s sub-shell first, followed by the 3d electrons when forming positive ions. This versatility enables them to participate in redox reactions, serving as powerful oxidizing or reducing agents, a principle demonstrated through key quantitative analytical techniques like redox titrations. Specific redox systems detailed include the use of manganate(VII) ions (MnO4 minus) with iron(II) ions (Fe2 plus) or ethanedioate ions (C2O4 2 minus), and the reaction between copper(II) ions (Cu2 plus) and iodide ions (I minus). Transition elements also function effectively as catalysts by temporarily changing their oxidation states or by accepting electron pairs into vacant d orbitals. Another defining property is the formation of complex ions through dative (coordinate) bonds with surrounding molecules or ions known as ligands. Ligands, classified by their denticity (monodentate, bidentate, or polydentate), determine the complex's coordination number and geometrical structure, which can be octahedral (six ligands), tetrahedral, square planar (four ligands), or linear (two ligands). These complexes can exhibit stereoisomerism, specifically geometric isomerism (cis/trans), such as seen in the anti-cancer drug cis-platin, and optical isomerism, particularly common in octahedral complexes involving bidentate ligands. The chapter also explores ligand exchange (substitution reactions), which occur when a newly formed complex is inherently more stable than the original. The relative stability of complexes is quantified by the stability constant, Kstab, where a larger value signifies a more stable complex. Crucially, the presence of ligands causes the degenerate d orbitals in the central metal ion to split into two non-degenerate energy levels. The energy difference (Delta E) between these levels corresponds to frequencies in the visible spectrum. Colour arises because the complex absorbs light energy (a specific complementary color), causing an electron jump, and the transmitted light determines the observed color. The type of ligand dictates the extent of this orbital splitting, thereby determining the complex's specific color.